This invention relates to signal monitoring devices and especially to such devices useful in monitoring power consumption level. There is presently a need for monitoring devices which provide an instantaneous indication of the input signal level. A particular example of this is the monitoring of power consumption levels to facilitate load shedding so as to maintain power consumption below a certain maximum level.
Various types of devices have been utilized in the past to monitor power consumption. Recording devices are available which record power consumption data on charts. But this type of device is generally only useful for a recording of historical data and does not provide an instantaneous output signal which can be used in a load shedding application.
The use of electronic watt meters has been the standard method for obtaining energy consumption data for large power consumers. While many of these devices are capable of providing an instantaneous output signal, they have many shortcomings. Electronic watt meters generally require current sensing transformers installed in the power lines to supply the watt meter with line current information. In addition, line voltage amplitude must be sensed to provide a voltage component necessary for power computation. Installation of the transformer and voltage sensing means is frequently expensive and in some instances is not feasible.
It is desirable to provide a way to monitor power consumption which does not involve the installation of additional auxiliary equipment in the power lines. Known power demand meters utilized by the utility companies for determining power usage are capable of providing a pulse signal which is representative of the rate of power consumption. By providing a monitoring device capable of decoding such a pulse signal the need for installation of auxiliary equipment in the power lines is eliminated. Two methods of accomplishing this decoding are time-to-voltage conversion and frequency-to-voltage conversion. A basic shortcoming of time-to-voltage conversion systems is that the output signal does not exhibit satisfactory linearity characteristics. While it has been found that frequency-to-voltage conversion devices generally exhibit better linearity, problems are encountered in applications in which the pulse signal is of a relative low frequency. This situation is encountered in monitoring power consumption since the demand meter pulse rates are generally quite low (typically 8-500 pulses per minute).
Another characteristic which is desirable in signal monitoring devices of this type is that it be adaptable to various installations in which the output signal being monitored may reach different maximum levels. For example, in power consumption monitoring applications one power utilization facility may consume power at one maximum rate while another such facility consumes power at a different maximum rate. It is desirable to provide a monitoring device suitable for use in facilities having different maximum consumption rates. In such cases the output signal of the monitoring device could be sent to a meter which provides a reading as a percent of the maximum power consumption.